1. Field of the Invention
This invention relates to high critical temperature, ceramic, oxide superconductors comprising one transition metal, one metal of Group 2 of the Periodic Table and one metal of Group 1 of the Periodic Table. A key distinctive feature of the products of this invention is the hexagonal crystal symmetry of their structure that comprises highly covalent oxide chains containing the transition metal. The chains are parallel to the c axis. More specifically the superconductors of this invention comprise the transition metals nickel or cobalt. They may be prepared in powder form, in polycrystalline compacts, in dense polycrystalline aggregates and in single crystals.
The invention also relates to processes to prepare the superconductors in each of the above mentioned forms.
In another specific embodiment the invention relates to precursors of the superconductors and their preparation.
Superconductors are useful materials that find applications in magnetic, electric and electronic applications such as high flux electromagnets, magnetic instrumentation, transmission lines, levitation phenomena, storage of electrical energy, etc.
2 Description of the Previously Published Art
2.1 High Critical Temperature Ceramic Oxide Superconductors.
Superconductivity is the property of materials that exhibit zero electrical resistance when cooled to or below a temperature called the critical temperature (Tc). It was discovered by H. Kamerlingh Onnes in 1911 using the extremely low temperature of liquid helium. For many decades superconductivity remained a laboratory curiosity with no extensive practical applications because of the very low temperatures required to achieve it in metals, metallic alloys and binary metallic compounds.
In the 1970""s superconductivity was observed in perovskite metal oxides structures. By 1975 A. W. Sleight and coworkers found that in BaPbO3 (barium plumbate) which is a perovskite-type oxide, 5 to 30% substitution of Bi for Pb induces superconductivity. These findings did not receive much attention from the scientific community possibly because of their low Tc.
In 1986 a major breakthrough was achieved by T. G. Bednorz and K. A. Mxc3xcller (Z. Phys. 1986, B64, 189) with the discovery of a complex ceramic oxide that becomes a superconductor at about 30 K. The material was a complex oxide of lanthanum, barium and copper, with perovskite-related symmetry (tetragonal with space group l4/mmm) and having a composition of
La1.85Ba0.15CuO4xe2x80x83xe2x80x83Formula 1
This impressive result was immediately followed up by much research throughout the world, and by 1987 the isostructural superconductor La1.85Sr0.15CuO4 with a Tc of about 37 K was prepared by J. M. Tarascon et al. (Science, 1987, 235, 1373). Also that year C. W. Chu et al (Phys. Rev. Lett. 58, 1891-1894 (1987)) prepared the superconductor YBa2Cu3O7 (called 1-2-3 because of the atomic ratios of the metals) with a Tc of about 93 K, which is higher than the boiling point of liquid nitrogen. This development was both a major scientific and a technological breakthrough because superconductivity was achieved for the first time using a practical and readily available coolant that opened a wide field of applications.
During the last 15 years, scientists have made many variations of, and advances over the original material, with increases in Tc to about 128 K (Tl2Ca2Ba2Cu3O10).
In order to clearly describe materials and avoid confusion, the specific meaning of certain terms used in this application, will be defined next.
The term xe2x80x9cparentxe2x80x9d is used to refer to oxide compounds consisting chemically of a transition metal oxide and an ionic metal oxide such as for example La2CuO4.
The term xe2x80x9cmain cationxe2x80x9d refers to the ionic metal cation in the parent material such as for example La3+.
The term xe2x80x9cdopingxe2x80x9d and related terms such as xe2x80x9cdopantxe2x80x9d, xe2x80x9cdopedxe2x80x9d, etc., refer to the replacement in the crystal structure, on a one for one atomic basis, of part of the main cation, by cations of different but fix valence for example Ba2+ in La1.85Ba0.15CuO4.
The new material discovered by Bednorz and Mxc3xcller lead to the new superconductor class referred to as high-Tc ceramic, oxide superconductors. The series of complex oxides prepared in their work may be visualized as descendants from the parent oxide lanthanum cuprate (La2CuO4) after doping with barium. The series may be represented by the variable formula:
La(2xe2x88x92x)BaxCuO4xe2x80x83xe2x80x83Formula 2
in which x ranges from about 0.05 to about 0.25. In this range the doping, either with Ba or Sr does not affect the tetragonal symmetry (space group l4/mmm) of the parent lanthanum cuprate, although it slightly changes the unit cell dimensions. The most significant discovery was that starting in the range of about x=0.05 to about x=0.10 the doped materials became low Tc superconductors. As x increased so did Tc up to x=0.15. At this point the Tc became about 30 K. Beyond x=0.15 the materials, while remaining superconductors, decreased in Tc.
Detailed studies of the crystal structure revealed that all the superconductors of the barium series, as well as the isostructural members of the strontium series consist of alternating charged layers of opposite sign. One type of layer is a covalent square planar array with a composition of [CuO4/2] or [CuO2] located next to parallel ionic layers with a composition of [La(2xe2x88x92x)BaxO2]. The average charge density of the ionic layer is readily calculable from the composition and valence of its constituent ions. It is +(2xe2x88x92x) per [La(2xe2x88x92x)BaxO2]. For the [CuO2] covalent layer the average charge density must become xe2x88x92(2xe2x88x92x) in order to maintain overall crystal neutrality.
It is interesting to note that the charge density of the [CuO2] covalent layer goes from xe2x88x922 for the parent material La2CuO4 to xe2x88x92(2xe2x88x92x) for any of the doped members of the series and that the [CuO2] layer undergoes oxidation as x increases. In this progression the parent material undergoes a transition from insulator to superconductors. The resulting Tc""s are in some unknown manner a function of the degree of oxidation of the covalent [CuO2] plane.
Work to better understand these observations lead to some fundamental questions regarding the oxidation of the [CuO2] layers.
Does it result in the oxidation of Cu2+ to Cu3+ or in the oxidation of O2xe2x88x92 to O1xe2x88x92 to form electron holes?
Is it possible that both elements undergo oxidation and that there exist an equilibrium between the four possible valences? (Cu2+, Cu3+, O2xe2x88x92 and O1xe2x88x92).
In the latter case, which one is the predominant oxidized element, Cu3+ or O1xe2x88x92?
This matter was studied using Hall effect measurements and other techniques that showed that the great majority of the electrical carriers are O1xe2x88x92 holes.
Regarding electrical conductivity, the present accepted view is that the holes move in the two-dimensional [CuO2] layers.
These data are in sharp contrast with the behavior of earlier metallic superconductors in which electrons carried the current in three, not two dimensions.
On another matter, the negative results of hundreds of studies done all over the world over the last 15 years or so using transition metals other than copper lead to the empirical xe2x80x9cinferencexe2x80x9d that only ceramic materials comprising copper oxide yield high Tc superconductors.
Another interesting point worth noting on the work of the last 15 years or so is that during that period no high-Tc, xe2x80x9cone-dimensionalxe2x80x9d ceramic oxide superconductor has been reported.
From a theoretical point of view, the fundamental reasons for the cuprates to become high-Tc superconductors are not as yet fully understood. The BCS (Bardeen, Cooper and Schrieffer) theory, which accounts for superconductivity in metals, metal alloys and binary metal compounds in three dimensions does not account for the high-Tc superconductors based on [CuO2] that conduct electricity in only two dimensions.
2.2. The Ceramic Barium Oxide-Nickel Oxide System
The following is a brief review of the work on the ceramic BaOxe2x80x94NiO system for materials with a Ba to Ni atomic ratio of 1.
None of the prior art involves superconductors.
The significant references follow.
The Phase System BaOxe2x80x94NiO. J. J. Lander (J. Amer. Chem. Soc. 1951, Vol. 73, p. 2450). (Ref. Lander JACS)
Barium-Nickel Oxides with Tri- and Tetravalent Nickel. J. J. Lander and L. A. Wooten (J. Amer. Chem. Soc. 1951, Vol. 73, p. 2452). (Ref. Lander et al. JACS)
The crystal Structure of NiO.3BaO, NiO.BaO, BaNiO3 and Intermediate Phases . . . J. J. Lander xe2x80x9cActa Cryst. (1951) 4, 148. (Ref. Lander Acta. Cryst.)
The Crystal Structure of BaNiO3. Takeda et al. Acta Cryst. (1976) B32, 2464. (Ref. Takeda 1976)
The work of Lander et al. included a series of compositions ranging between BaNiO2 (divalent nickel) and BaNiO3 (tetravalent nickel).
The crystal structure of BaNiO2 was determined by x-ray diffraction (XRD) techniques (rotation photographs, back-reflection Laue patterns and powder patterns). It is orthorhombic but the Laue pattern obtained with the radiation parallel to the co axis was found to have six-fold symmetry. Moreover the ratio of bo to ao was close to the value of 3 indicating a pseudo-hexagonal structure.
The crystal structure of BaNiO3 was determined by the powder XRD method and its pattern was readily indexed as hexagonal. Table I shows the key structural parameters of the BaNiO2 and BaNiO3 crystalline phases.
Three other materials were prepared with intermediate compositions between the two end products. They reported data for the reversible sequence shown next:
BaNiO3⇄Ba4Ni4O11⇄Ba3Ni3O8⇄Ba2Ni2O5⇄BaNiO2
which may be also written as:
BaNiO3⇄BaNiO2.75⇄BaNiO2.67⇄BaNiO2.5⇄BaNiO2
The structures of the three intermediates were not determined although they were reported to be hexagonal or nearly hexagonal similar to the symmetry of BaNiO3. The sequence from left to right may be brought about by increasing the temperatures during synthesis or post treatments at constant oxygen pressure which indicates a tendency to lose oxygen by dissociation.
In the opposite direction the sequence showed oxidation of the nickel from Ni2+ to Ni3+ to Ni4+ including mix-valence compounds as the synthesis temperature decreases.
Important points noted by the various authors and found out presently are:
All the intermediates exhibit hexagonal or nearly hexagonal symmetry and have almost identical hexagonal unit cells when viewed on the c direction with the barium ions in a hexagonal closed-packed arrangement like that observed in BaNiO3.
The five member sequence may be represented by the formula BaNiO2+y (0xe2x89xa6yxe2x89xa61).
The members of the sequence for example 0xe2x89xa6y less than 0.9 are precursors to BaNiO3.
Single crystals of BaNiO2 were easily grown in a molten BaCl2 flux at about 1000xc2x0 C. in nitrogen.
BaNiO3 can be prepared by reacting:
A. BaO2 and NiO quantitatively in an atmosphere of wet O2 at 450-700xc2x0 C. to form the crystal structure of BaNiO3 but achieving only 98% oxidation. Beyond about 730xc2x0 C. oxygen dissociation takes place.
B. Ba(OH)2 with NiO in a wet atmosphere of O2 at about 700xc2x0 C. to form the crystal structure of BaNiO3 but achieving only 90% oxidation.
Takeda et al. (1976) proceeded to react BaCO3 with NiO in air at 1100xc2x0 C. for 48 hours to form an intermediate that was most likely a member of the Lander et al. sequence of the general formula BaNiO2+y. The chemical analysis of this product was not reported nor was any XRD data given. It was obviously a precursor because subsequent annealing at 600xc2x0 C. in pure oxygen at an absolute pressure of 2000 bars formed the product BaNiO3.
The resulting polycrystalline powder was converted into single crystals using a Ba(OH)2.8H2O flux at 600xc2x0 C. under an absolute oxygen pressure of 2000 bars. A single crystal was used to determine the highly refined crystal structure of BaNiO3. A picnometer determination of its density gave 6.10 g/cm3 which corresponds to a composition of BaNiO2.99 and a nickel valence of +3.98. The data of Lander and Takeda et al. are given in Table II.
The most significant findings reported by Lander and Takeda et al. on BaNiO3 were:
The octahedral coordination of oxygen around nickel.
The unusual arrangement of the nickel-oxygen octahedra which are stacked in columns or chains along the c axis sharing facets as shown in FIG. 1A.
All nickel-oxygen bonds are highly covalent and the composition of each octahedron is xe2x80x94[NiO6/2]xe2x80x94 or xe2x80x94[NiO3]xe2x80x94.
The nickel exhibits its highest valence (+4) or virtually so and the charge of each octahedron is xe2x88x922 or virtually so.
The chains run the full length of crystallites or single crystals along the c axis. They are called polyacids by Takeda (1976). The term macroanions is preferred in the present disclosure.
The electrical charge of the macroanions or chains is neutralized by one Ba2+ per octahedron, as shown in FIG. 1B.
Hexagonal symmetry with space group P63/mmc as shown in the unit cell representation given in FIG. 2 that gives five sections at regular intervals along the c axis.
An important feature of the structure that may be calculated from the detailed results given by Takeda et al. is the separation of the chains (5.629 xc3x85 axis to axis). Furthermore, it can be calculated that the oxygen to oxygen distance between nearest oxygen atoms from adjacent chains is too large for orbital overlap or mixing.
2.3. Related Crystal Structures
Table III in FIG. 3 shows the XRD data of barium nickelate (Takeda et al 1976). Acta Crystallogr., Sec. B. 32, 2464 (1976)); barium cobaltate (Takeda et al. JINCAO; 34, 1599; 1972), and strontium nickelate (Tagushi, H. et al., Acta Crystallogr., Sec. B, 33, 1298 (1977)). The three crystalline phases are isostructural, hexagonal, (with space group P63/mmc) showing only minor differences in their unit cell parameters.
All these crystalline phases with different transition metals and ionic cations constitute the starting parent materials from which several series of doped product can be prepared.
The overall objective of this invention is the synthesis of one-dimensional oxide superconductors based on transition metals other than copper.
It is a further object of this invention to prepare:
1. One-dimensional superconductors comprising linear, parallel, covalent chains of transition metal oxides held together by highly ionic cations. The superconductors exhibit hexagonal crystalline symmetry.
2. Polycrystalline powders, compacts and dense bodies, of object 1.
3. Single crystals from the superconductors of objects 1 or 2.
4. Polycrystalline precursors to objects 1 and 2.
It is a further object of this invention to develop methods of preparation for the materials, described in objects 1 through 4.
These and further objects of the invention will become apparent as the description of the invention proceeds.
In its broadest aspect, the new compositions of this invention are one-dimensional, ceramic oxide, crystalline superconductors comprising parallel, highly covalent chains or macroanions consisting of octahedrally coordinated oxygen around the transition metals that exhibit their highest formal valence. The chains or macroanions are held together by highly ionic, bulky cations of Group 2 (main cation) and Group 1 (doping cation) of the Periodic Table and these single valence bulky cations are capable of inducing the formation of electrical carriers (holes) within these chains or macroanions.
1. Compositions and Methods for Making.
A crystalline superconducting composition is made having the formula
M2+(1xe2x88x92x)M1+xMTO3xe2x88x92xcex4xe2x80x83xe2x80x83Formula 3
where
M2+ is a main cation of Group 2 of the Periodic Table,
M1+ is a doping cation of Group 1 of the Periodic Table,
MT is the transition metal Ni or Co,
0.04 less than xxe2x89xa60.30,
0xe2x89xa6xcex4xe2x89xa60.20, and
having hexagonal symmetry, preferably with space group P63/mmc. M2+ is preferably Ba2+ or Sr2+; M1+ is preferably K1+, Na1+, or Rb1+; x is preferably 0.05xe2x89xa6xxe2x89xa60.20; and
xcex4 is preferably 0xe2x89xa6xcex4xe2x89xa60.10.
2. The Superconductor Compositions may be Prepared by Two General Methods.
2.1 A Direct Method in which:
Oxides or hydroxides of M2+ and M1+ are mixed together with oxides of MT in the desired metals proportions and heated under flowing oxygen to a temperature not to exceed about 700xc2x0 C. such as for example:
2.1.1 The hydroxides of the two metals M2+ and M1+ are reacted with an oxide of the transition metal MT under an oxygen atmosphere at about 600xc2x0 to 700xc2x0 C.
2.1.2 The peroxides of the two metals M2+ and M1+ are reacted with an oxide of the transition metal MT under an oxygen atmosphere at temperatures ranging from about 450xc2x0 C. to about 700xc2x0 C.
2.1.3 Synthesis Using M2+ Peroxides and M1+ Superoxides.
The most desirable direct methods of preparation are those in which all the reactants possess the highest atomic oxygen to metal ratio such as MT2O3, M2+O2 and M1+O2. These reactants possess more than the stoichiometric requirements of oxygen needed to oxidize the transition metal to +4 while bringing very favorable low melting points that facilitate complete reaction at low temperatures.
2.2 The Two-step Method Starts with the Preparation of a Precursor with the Composition:
xe2x80x83M2+(1xe2x88x92x)M1+xMTO2+yxe2x80x83xe2x80x83Formula 4
where M2+, M1+, MT and x are defined in Formula 3, and 0xe2x89xa6yxe2x89xa60.90.
In essence the precursors are prepared from compounds M2+, M1+, MT that yield the oxides such as, for example, the carbonates or mixtures of the carbonates with oxides at high temperatures in air or oxygen.
The precursors exhibit hexagonal or nearly hexagonal symmetry. They may be oxidized to the highest attainable valence of the transition metal MT to prepare the superconductor composition under very high absolute pressures of oxygen and at temperatures greater than about 500xc2x0 C.
In the two step method the precursor composition of the Formula 4
M2+(1xe2x88x92x)M1+xMTO2+y
may be made by mixing compounds, other than oxides, of the three metallic reactants M2+, M1+, and MT in the required proportions and thermally treating the mixture in flowing air or oxygen at sufficient temperature to decompose and oxidize if necessary the compounds to obtain the oxides and react them to form the precursor compositions that often exhibit hexagonal symmetry.
3. Preparation of Single Crystals of the Superconductors.
The superconducting compositions may be prepared as single crystals from polycrystalline superconducting powders of the Formula 3 using a flux of M2+(OH)2.8H2O and M1+OH at very high absolute pressures of oxygen at temperatures greater than 500xc2x0 C.
4. Crystal Structure
The crystal symmetry of the superconductors is hexagonal preferably with space group P63/mmc. The symmetry of any of the materials of this class is the same as the symmetry of their parent material. However, the unit cell edges will likely be somewhat different. The difference will be small because of the relative low doping levels used to achieve electrical conductivity.
5. Electrical Conductivity
For x=0 the materials are virtually electrical insulators. For values of x greater than about 0.04 to about 0.05 the materials become conductors and in the range of x of 0.05 or higher there is a conductivity transition to hole type superconductors.
The holes travel within highly covalent, tubular-like macroanions of about 0.5 nanometers in diameter. They run parallel to the c crystal axis and because the nearest distance between adjacent macroanions is too large for orbital overlap, these crystalline products are one-dimensional superconductors.